1. Field of the Invention
The present invention relates to a cell evaluation device for evaluating the safety of a cell on the basis of cell information obtained by causing an intentional internal short circuit using either a nail penetration test or a crush test.
2. Description of the Related Art
In cells such as rechargeable lithium ion cells that use a flammable organic electrolyte, although safety functions such as protection circuits are provided to ensure that abuse of the cell or faults within the connected equipment do not cause overheating within the cell, these safety functions are not designed to cope with extreme examples of abuse. However, safety standards are set so that even in cases of extreme abuse, thermal runaway does not lead to explosion or ignition of the cell. Such safety standards demand that even in cell evaluation tests conducted under the most severe conditions, explosion or ignition of the cell does not occur. These cell evaluation tests are prescribed by the UL standard used for lithium cells (UL1642), and within guidelines issued by the Japan Storage Battery Association (see “Guidelines for safety evaluations of rechargeable lithium cells,” G1101-1997).
Test items are broadly classified into electrical tests, mechanical tests, and environmental tests. Within the mechanical tests that represent the target for the present invention, abuse tests include the nail penetration test and the crush test. In the case of the nail penetration test, a fully charged fresh cell, and a cell that has fallen to half its rated capacity are used as test cells, and a nail is punched perpendicularly through the electrode surfaces from a point near the middle of the sidewall of each test cell. The standard requires that neither explosion nor ignition occurs, even if the test cells are left in this punctured state for 6 hours or longer.
However, conventional nail penetration and crush tests simply specify the insertion of a nail either right through, or to a certain depth within, the test cell, or the crushing of the cell with a predetermined pressure respectively. Accordingly, the location of the short circuit generated by the nail penetration or the crushing varies from test to test, meaning the short circuit location cannot be specified, and the temperature variation arising from Joule heating caused by the short circuit current is not stable.
Furthermore, the size of the short circuit current that flows as a result of the internal short circuit caused by the nail penetration or the crushing is determined by the internal resistance of the cell. Consequently, during the evaluation of different cells, the Joule heating caused by the short circuit current, and the heating caused by chemical reactions between the component materials of the cell, differ considerably, meaning accurately determining the cause of cell temperature variations has been impossible. In addition, in those cases where an internal short circuit occurs within one cell of a battery pack or the like, where a plurality of cells are connected in parallel, increases in the cell temperature caused by short circuit current flowing from other cells within the parallel connected pack have been unable to be evaluated. Moreover, when evaluation is required for a cell in which cycle degradation or high temperature degradation has caused an increase in internal resistance, a cycle degraded cell or high temperature degraded cell must first be prepared for the purposes of testing.